Natural gas liquefaction

ABSTRACT

The invention relates to a method for liquefying a hydrocarbon-rich feed fraction, preferably natural gas, against a nitrogen refrigeration cycle. A feed fraction is cooled against gaseous nitrogen that is to be warmed, and liquefied against liquid nitrogen that is to be vaporized. The feed fraction is cooled and liquefied in an at least three-stage heat-exchange process. In the first section of the heat-exchange process, the feed fraction is cooled against superheated gaseous nitrogen to the extent that an essentially complete separation of the relatively heavy components is achievable. In the second section, the feed fraction freed from relatively heavy components is partially liquefied against gaseous nitrogen that is to be superheated. In the third section, the feed fraction is liquefied against nitrogen that is to be partially vaporized.

SUMMARY OF THE INVENTION

The invention relates to a method for liquefying a hydrocarbon-rich feedfraction, preferably natural gas, against a nitrogen refrigerationcycle, wherein the feed fraction is cooled against gaseous nitrogen thatis to be warmed and the feed fraction is liquefied against liquidnitrogen that is to be vaporized.

Hydrocarbon-rich gases, in particular natural gases, are liquefiedcommercially in a capacity range from 10 to 30,000 tons of LNG per day(tpd). In plants of medium capacity—this is taken to mean liquefactionprocesses having a capacity between 300 and 3000 tpd of LNG—and largecapacity—this is taken to mean liquefaction processes having a capacitybetween 3000 and 30,000 tpd of LNG—those skilled in the art attempt tooptimize the operating costs by means of high efficiency. In contrast,in the case of smaller plants—this is taken to mean liquefactionprocesses having a capacity between 10 and 300 tpd of LNG—low capitalcosts are in the foreground. In such plants, the capital cost proportionof a dedicated refrigeration plant in which the working medium used is,for example, nitrogen or a nitrogen-hydrocarbon mixture, isconsiderable. Therefore, generation of cold in the liquefaction plantis, if possible, dispensed with and a suitable refrigerant imported.Customarily, in this case, liquid nitrogen is used and after its use asrefrigerant, is given off to the atmosphere in the gaseous state. If innearby air separation plants unused product amounts of liquid nitrogencan be provided inexpensively, this concept for small liquefactionplants is absolutely commercially expedient.

For reasons of costs, in small liquid-nitrogen-cooled plants, brazedaluminum plate heat exchangers are generally used. These appliances,however, are sensitive to high thermal stresses as can arise, forexample, by an oversupply of refrigerant and/or large temperaturedifferences between warm and cold process streams. The resultantmechanical stresses can lead to damage to these appliances.

In addition, care must be taken to ensure that, during operation of theliquefaction process, the feed fraction does not fall below the freezingtemperature. The solid point of methane at —182° C. is markedly abovethe atmospheric boiling temperature of nitrogen, which is −196° C.Freezing of the plant always causes an unwanted operating fault and can,in addition, have lasting damage as a consequence.

A method of the type in question for liquefying a hydrocarbon-rich feedfraction is known from U.S. Pat. No. 5,390,499. This method is suitable,in particular, for plants of small capacity, as explained at the outset.In the liquefaction method described in U.S. Pat. No. 5,390,499, the gasto be liquefied is cooled and liquefied against nitrogen in two separateheat exchangers. In this case the liquid low-boiling nitrogen iscompletely vaporized in the second heat exchanger and warmed up to atemperature at which relatively heavy crude gas components can be takenoff in the liquid state by means of a separator from the gas that is tobe liquefied. In a process procedure as described in U.S. Pat. No.5,390,499, however, the point at which the nitrogen vaporizes completelycan vary considerably according to load. This can lead to unwantedprocess conditions which have the abovementioned disadvantages as aconsequence.

It is an object of the present invention to provide a method of the typein question for liquefying a hydrocarbon-rich feed fraction, whichmethod avoids the abovementioned disadvantages and, in particular, toprovide a method which is robust against operating faults and damage.

Upon further study of the specification and appended claims, otherobjects and advantages of the invention will become apparent.

For achieving these objects, a method is proposed for liquefying ahydrocarbon-rich feed fraction, which is characterized in that

-   -   the feed fraction is cooled and liquefied in an at least        three-stage heat-exchange process,    -   wherein, in the first section of the heat-exchange process, the        feed fraction is cooled against superheated gaseous nitrogen to        the extent that an essentially complete separation of the        relatively heavy components is achievable,    -   in the second section of the heat-exchange process, the feed        fraction freed from relatively heavy components is partially        liquefied against gaseous nitrogen that is to be superheated,        and    -   in the third section of the heat-exchange process, the feed        fraction is liquefied against nitrogen that is to be partially        vaporized.

The method according to the invention is suitable for use in plants oflarge (3000-30,000 tpd of LNG), medium (300-3,000 tpd of LNG), or small(10-300 tpd of LNG) capacities. The most economical capacity range,however, is 10-300 tpd of LNG.

As mentioned, the method according to the invention is directed toliquefaction of a hydrocarbon-rich feed fraction, such as natural gas.For example, the hydrocarbon-rich feed fraction can contain 80 to 99vol. % methane, 0.1 to 10 vol. % ethane, 0 to 5 vol. % propane, 0 to 4vol. % C4+hydrocarbons, 0 to 10 vol. % nitrogen, 0 to 10 vol. % carbondioxide, 0 to 1 vol. % hydrogen sulfide, up to trace amounts of othersulfur species, up to trace amounts of helium, and up to trace amountsof hydrogen.

The expression “heavy components” may be taken to mean ethane and highermolecular weight hydrocarbons.

Further advantageous embodiments of the method according to theinvention for liquefying a hydrocarbon-rich feed fraction arecharacterized in that

-   -   the three-stage heat-exchange process is achieved in one or more        heat exchangers,    -   the condensation pressure of the feed fraction freed from        relatively heavy components is adjusted to values between 1 and        15 bara (absolute pressure), preferably between 1 and 8 bara,        and    -   the boiling pressure of the gaseous nitrogen that is to be        superheated is adjusted to values between 5 and 30 bara,        preferably between 10 and 20 bara.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated schematically with reference to anexemplary embodiment in the drawing and will be described extensivelyhereinafter with reference to the drawing. Various other features andattendant advantages of the present invention will be more fullyappreciated as the same becomes better understood when considered inconjunction with the accompanying drawing wherein:

the FIGURE illustrates an embodiment according to the invention.

As shown in the FIGURE, the hydrocarbon-rich feed fraction that is to beliquefied is fed via line 1 to a heat exchanger E1. This is subdividedinto three sections or stages a to c. The boundaries between thesesections or stages are shown by the two dashed lines. In the warmestsection a of the heat exchanger E1, the hydrocarbon-rich feed fractionis cooled against superheated gaseous nitrogen, which is fed via line 9to the heat exchanger E1. Here, the hydrocarbon-rich feed fraction iscooled to the extent that a separation of the heavy components from thefeed fraction is possible in a separator D2 downstream of the heatexchanger E1. For this purpose, the cooled feed fraction is fed from theheat exchanger E1 via line 1′ to the separator D2. From the bottom phasethereof, via line 2′, in which a valve V1 is provided, the unwantedheavy components are removed in liquid form and discharged from theprocess.

Instead of separator D2 shown in the FIGURE, a rectification column canbe used to achieve a more precise separation of relatively heavycomponents or higher hydrocarbons from the feed fraction.

At the top of the separator D2, via line 2, the feed fraction, freedfrom heavy components, is removed and fed to the second section b of theheat exchanger E1. Therein, the feed fraction that is freed from heavycomponents is partially liquefied against gaseous nitrogen that is to besuperheated 9. Then, in the third stage c of the heat exchanger E1, thefeed fraction is completely liquefied against nitrogen to be partiallyvaporized which is fed to the heat exchanger E1 via the line 8.

The liquefied feed fraction, after passage through the heat exchanger E1is fed to a storage vessel D4 via line 3, in which a control valve V3 isarranged. The liquefied product (LNG) can be discharged therefrom vialine 4. The control valve V3 serves for expanding the liquefied feedfraction to the product delivery pressure, which corresponds at leastapproximately to atmospheric pressure.

If the nitrogen is vaporized in the third section c of the heatexchanger E1 at a pressure of greater than 15 bara, the boilingtemperature thereof is no longer low enough in order to subcool theliquefied feed fraction to the extent that outgassing after expansionthereof in the control valve V3 can be prevented. In this case, theboil-off gas formed in the storage vessel D4 is advantageously removedvia line 5, compressed in the compressor C3 and fed back to the feedfraction 2 which is freed from heavy components before liquefactionthereof and reliquefied in the heat exchanger E1. This process procedureshould be selected, in particular, in the case of significant temporarystorage of the LNG product in an atmospheric flat-bottom tank D4, sincethe resultant boil-off gas is also processed thereby.

The nitrogen required for providing cold is fed to the liquefactionprocess via line 6. Advantageously, a buffer tank D3 is provided whichserves for compensating for quantitative fluctuations of the feedfraction that is to be liquefied and/or of the refrigerant nitrogen. Bymeans of a pump P1, liquid nitrogen is fed in the amount required to aseparator D1 via line 7. From the bottom phase of the separator D1,boiling nitrogen is removed and conducted via line 8 through the coldestsection c of the heat exchanger E1. The nitrogen that is partiallyvaporized in this case is then fed via line 8′ back to the separator D1.

If the reliquefaction process that is still to be described is operated,at least temporarily the generation of cold by the reliquefaction of thenitrogen can exceed the refrigeration requirement of the natural gasliquefaction. An oversupply resulting therefrom of liquid nitrogen canbe delivered into the buffer tank D3 via line 8″ and valve V6.

At the top of the separator D1, gaseous nitrogen is taken off via line 9and fed to the middle section b of the heat exchanger E1. The gaseousnitrogen is conducted through the second and first sections of the heatexchanger E1 in countercurrent flow to the feed fraction 2 that is to becooled and partially liquefied, and is warmed and superheated in thisprocess. The superheated nitrogen is then removed from the process viathe line sections 10 and 11.

By means of the control valve V4, the boiling pressure of the gaseousnitrogen that is to be superheated 9 can be controlled. Advantageously,this boiling pressure is adjusted to values between 5 and 30 bara,preferably between 10 and 20 bara.

Similarly, the condensation pressure of the feed fraction 2 that isfreed from relatively heavy components can be controlled by means of thecontrol valve V2. This condensation pressure is preferably adjusted tovalues between 1 and 15 bara, preferably between 1 and 8 bara.

By means of the control valves V2 and/or V4, the temperature profile inthe third section c of the heat exchanger E1 can be controlled thereby.By means of the control valve V2, the condensation pressure of the feedfraction is established in the section between the control valves V2 andV3, and, by means of the control valve V4, the boiling pressure of thenitrogen in the separator D1 and the third section c of the heatexchanger E1 is controlled. Owing to the above-described subdivision ofthe heat-exchange process into a second and third section and with thephase separation in separator D1 it can then be established exactly inwhat section of the heat exchanger E1 a (partial) vaporization orsuperheating of the nitrogen is taking place.

By means of the subdivision of the heat-exchange process E1 into threesections a to c, it is possible to reliably prevent the phase boundarybetween liquid and gaseous refrigerant from migrating within the heatexchanger E1 and thereby causing unwanted thermal and mechanicalstresses within the heat exchanger E1.

If the nitrogen boiling pressure (pN₂) and the crude gas condensationpressure (pRG) are selected according to the inequality pRG (bara)≧0.3pN₂ (bara) −1, a thermal overload of the heat exchanger E1 due toimpermissibly high temperature differences can be safely avoided.

By restricting the boiling pressure of the liquid nitrogen in the thirdsection c of the heat exchanger E1 and of the separator D1 to at least 5bara—the associated boiling temperature is −179° C.—it is possible toprevent reliably a temperature below the freezing temperature of methaneoccurring in the heat exchanger E1. Operating problems and possibledamage due to solids formation are thereby excluded.

The superheated nitrogen taken off from the heat exchanger E1 via line10 can, alternatively to a removal via line 11, be at least partiallyreliquefied. For this purpose the nitrogen is fed via the line sections12 and 13 to a compression—shown in the figure by a two-stage compressorunit C1/C2, wherein a heat exchanger, E3 or E4 respectively, isconnected downstream of each compressor unit—and then is fed via line 14to a heat exchanger E2. Therein, the nitrogen is reliquefied and thenfed to separator D1 via line 15. Pressure regulation of the compressorC2 is performed by the control valve V5. For the purpose of providingcold in the heat exchanger E2, a substream of the compressed nitrogenstream is removed via line 16, preferably expanded in a multistagemanner—shown by the gas expanders X1 and X2—and then conducted via line17 through the heat exchanger E2 in countercurrent flow to the nitrogenstream that is to be liquefied. The shafts of the compressors C1 and C2are preferably coupled to the shafts of the gas expanders X2 and X1.

If the above-described reliquefaction process is operated, it isadvantageous to feed to the heat exchanger E1 via line 9 only the amountof gaseous nitrogen that is required for a small positive temperaturedifference of approximately 3° C. between streams 1 and 10 at the warmend of the heat exchanger E1. The excess amount of cold gaseous nitrogenis used via line 9′ proportionately for reliquefaction in the heatexchanger E2.

In principle, the liquefaction process can proceed by means of“imported” nitrogen—in this case, the superheated nitrogen is taken offfrom the heat exchanger E1 via the line sections 10 and 11—by means ofreliquefied nitrogen, or by any desired combination of both modes ofoperation.

The entire disclosure[s] of all applications, patents and publications,cited herein and of corresponding German Application No. DE 10 2010044869.9, filed Sep. 9, 2010 are incorporated by reference herein.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

1. A method for liquefying a hydrocarbon-rich feed fraction against anitrogen refrigeration cycle, comprising: cooling said wherein feedfraction against gaseous nitrogen that is to be warmed, and liquefyingsaid feed fraction against liquid nitrogen that is to be vaporized,wherein said feed fraction is cooled and liquefied in an at leastthree-stage heat-exchange process (E1 a-E1 c), in the first section ofsaid heat-exchange process (E1 a), said feed fraction (1) is cooledagainst superheated gaseous nitrogen (9) to the extent that anessentially complete separation (D2) of relatively heavy components (2′)is achievable, in the second section of said heat-exchange process (E1b), the feed fraction (2) freed from relatively heavy components ispartially liquefied against gaseous nitrogen that is to be superheated(9), and in the third section of said heat-exchange process (E1 c), thefeed fraction (2) is liquefied against nitrogen that is to be partiallyvaporized (8).
 2. The method according to claim 1, wherein saidhydrocarbon-rich feed fraction is natural gas.
 3. The method accordingto claim 1, wherein said three-stage heat-exchange process (E1 a-E1 c)is performed in one heat exchanger.
 4. The method according to claim 2,wherein said three-stage heat-exchange process (E1 a-E1 c) is performedin one heat exchanger.
 5. The method according to claim 1, wherein saidthree-stage heat-exchange process (E1 a-E1 c) is performed in more thanone heat exchanger.
 6. The method according to claim 2, wherein saidthree-stage heat-exchange process (E1 a-E1 c) is performed in more thanone heat exchanger.
 7. The method according to claim 1, wherein thecondensation pressure of the feed fraction (2) freed from relativelyheavy components is adjusted (V2) to a value of 1-15 bara.
 8. The methodaccording to claim 7, wherein the condensation pressure of the feedfraction (2) freed from relatively heavy components is adjusted (V2) toa value of 1-8 bara.
 9. The method according to claim 1, wherein theboiling pressure of the gaseous nitrogen that is to be superheated (9)is adjusted (V4) to a value of 5-30 bara.
 10. The method according toclaim 9, wherein the boiling pressure of the gaseous nitrogen that is tobe superheated (9) is adjusted (V4) to a value of 10-20 bara.